It is difficult to detect abnormal concave-convex irregularities in a workpiece such as a steel plate, a resin molded component, etc., having a rough surface by which an irradiation light is reflected in dispersion. For example, in the case of a steel plate, a method exists in which a streak caused at a time of rolling is first detected and then concave-convex irregularities are detected by irradiating lights in parallel, perpendicular to the streak. However, there is a limitation in the accuracy of such detection.
Meanwhile, an inspecting apparatus capable of detecting defects (e.g., abnormal concave-convex irregularities) in a surface of a workpiece with high accuracy is disclosed in JP2002-323454A, in which a plurality of reflected images of a stripe pattern are detected with a phase shift. In such a case, the abnormal concave-convex irregularities are detected by applying a phase shift method to the plurality of detected images.